When a customer wants to acquire a pair of corrective spectacles at an opticians, he starts by choosing a spectacle frame in which dummy demonstration lenses are mounted.
The optician then needs to trim ophthalmic lenses to replace these demonstration lenses.
For each ophthalmic lens, the optician carries out four main operations, which are:                acquiring the outline of one of the eyewires of the selected spectacle frame;        centering the lens, which consists in determining the optical frame of reference of this lens then in transferring to this ophthalmic lens the outline acquired beforehand, in such a way that, once trimmed to this outline and mounted in the spectacle frame, the lens is suitably centered facing the pupil of the corresponding eye of the customer;        blocking the ophthalmic lens, which consists in fastening a blocking accessory to the front face of the lens in a determined position, in order to allow the lens to be handled without misplacing the frame of reference of this lens (this being necessary when the lens must be passed from one machining station to another); then        machining the lens, which consists in cutting it to the desired outline.        
In the context of the present invention, spectacle frames with eyewires, i.e. rimmed spectacle frames, are more particularly of interest.
The machining operation then comprises a roughing step in which the initial contour of the lens is brought closer to the desired outline, then a beveling step that allows a fitting rib (commonly called a bevel) to be formed on the edge face of the lens, which fitting rib is able to fit into a fitting groove (commonly called a bezel) that runs along the inner face of the corresponding eyewire of the frame.
The acquiring and machining operations must be carried out with care if the lens is to fit perfectly into its eyewire, without effort and “the first time round”, i.e. without additional machining (rework).
To acquire the shape of the eyewire, an outline-reading apparatus including a surface profilometer that is slid along the bezel of the eyewire directly is generally used.
This reading apparatus then allows the shape of the bottom of the bezel to be registered in order to allow the lens to be machined in such a way that the top of its bevel has a homologous shape to that of the bezel.
The major drawback of this solution is that it requires an outline reader to be used, the purchase and maintenance costs of which are high.
It is moreover time-consuming to implement and requires great care to be taken by the optician, this running contrary to the sought-after principle of simplicity.